High-frequency bridge circuit



Oct. l1, 1949.

Filed Feb. 17, 1945 W. W. HANSEN HIGH-FREQUENCYl BRIDGE CIRCUIT 2 Sheets-Sheet l Figi.

INVENTOR W/L L /AM W. /QA/SEN ATTORN EY @CL 1i, 1949. W W, HANSEN 2,484,028

- HIGH-FREQUENCY BRIDGE CIRCUIT Fired Feb. 17, 1945 2 sheets-'sheet 2 TTORNEY atented ct. Il, 194g UNITED STATES ATENT OFFICE HIGH-FREQUENCY BRIDGE CIRCUIT William W. Hansen, Garden City, N. Y., assignor to rIhe Sperry Corporation, a corporation of' Delaware 8 Claims.

The present invention relates to apparatus for the conduction and/or transmission of ultra high frequency energy. More particularly, the present invention is concerned with apparatus for enicient energy coupling between a balanced source and an unbalanced load, or between an unbalanced source and a balanced load.

In dealing with ultra high frequency circuits, it is often desirable to provide an ultra high frequency voltage which is substantially balanced or symmetrical with respect to ground. This often presents a problem, since many types of ultra high frequency generators are arranged for providing an unbalanced output circuit as, for example, an output circuit supplied through a coaxial transmission line wherein a tubular shield at substantially ground potential forms one conductor and the inner wire at relatively high voltage is the other conductor. In such systems, of course, the asymmetrical nature of the coaxial line often proves disadvantageous. For example, it has been found that in cases where the inner conductor of a coaxial line is connected to one of the one-fourth wavelength elements of a dipole antenna and the outer conductor or shield of the coaxial line is connected to the other of the one-fourth wavelength elements of the antenna, unsatisfactory performance of the dipole antenna results from unequal excitation of the two quarter-wave elements.

Accordingly, it is an object of the present invention to provide an ultra high frequency transformer for coupling an unbalanced transmission line or device to a balanced transmission line or device.

It is another object of the present invention to provide ultra high frequency transformer apparatus for receiving energy from an unbalanced input circuit and providing a balanced output circuit.

It is another object of the present invention to provide ultra high frequency transformer apparatus for receiving energy from a balanced input circuit and providing an unbalanced output circuit. n

A further object is to provide a device for coupling a dipole antenna to an unbalanced transmission line in such a manner as to provide balanced excitation of quarter-wave elements of the dipole antenna.

In accordance with a feature of the present invention, a reactance bridge circuit is provided in which alternate elements around the ring circuit of the bridge are inductive and capacitive react- ,ance elements. Such elements may be provided in compact, easily constructed form by the use of transmission-line sections of suitable length, each transmission-line section being short-circuited at one end and connected at its opposite or input end in the ring circuit of the bridge. The lengths of the transmission-line sections between their short-circuited ends and their input ends are adjusted to provide the desired reactances at the operating wavelength of the transformer.

The foregoing objects and general description of the present invention will become clear and other objects will b-ecome apparent from the following description of the present invention, taken in conjunction with the drawings, wherein:

Fig. 1 is a sectional plan view, Fig. 2 is a front Y elevation and Fig. 2A is a further sectional detail view of a transformer constructed in accordance with the present invention, and employed to couple a coaxial transmission line to a dipole antenna;

Fig. 3 is a schematic diagram illustrating the application of parallel-conductor transmissionline sections to the transformer bridge arrangement of the present invention;

Fig. 4 is a reactance bridge diagram for illustrating the principles of the structures illustrated in Figs. 1 to 3, and Fig, 4A is a vector diagram showing the voltage relations therein; and

Fig. 5 is a front elevation, partly in section, of a transformer generally similar to that of Figs.

, 1 and 2, employed for coupling a coaxial transmission line to a balanced transmission line having two parallel conductors.

Referring now to Figs. 1, 2 and 2A, a source of energ-y il is-shown connected between the inner conductor I2 and the tubular cuter conductor i3 of a coaxial transmission line ifi, for supplying ultra high frequency energy to the quarter-wave portions l5 and it of a dipole antenna. lIhe tubular outer conductor i3 usually is grounded, as shown at l. According to common practice, one

` element i5 of the dipole antenna i5, ifi might be connected to the outer conductor I3 of the transmission line It, while the other quarter-wave portion It of the dipole antenna might be connected to the inner conductor i2 of the transmission line It. As noted above, however, such a connection of quarterawave portions of a dipole antenna directly to the inner and outer conductors of a coaxial transmission line often results in inequality of excitation of the quarter-wave antenna elements, and thus in impaired performance of the antenna.

, In accordance with the present invention, a compact transformer Il is provided for receiving the asymmetrical excitation from source II' through the coaxial line I4 and providing accurately balanced excitation to the elements I5 and It of the half-wave dipole antenna. The transformer Ii' comprises a tubular outer conductor I8 having two diametrically opposite openings I9 and i midway between the ends thereof, for clearance of the dipole antenna I5, I6. Between the openings I9 and `2l there is provided a further opening 22 for receiving the outer tubular conductor I3 of the coaxial transmission line I4. A rigid, electrically conductive junctionof -the tubular outer conductor I8 and the tubular outerconductor i3 is provided, as bysoldering kthese elements together at the opening 22.

A conductive member 24 having a disc end 25 and an axially extending :portion 26 .having a length of substantially three-eighths the .wavelength of source II isprovided within one end of the tubular outer conductor I8 of transformer I'I. A cylindrical recess 21 is formed in the end of extension of the member 24 to a depth substantially equal to one-eighth the'wavelength of source I I. Another conductive member 3I is provided in the opposite end of the tubular outer conduotor I8. The conductive member 3IV has bored therein a cylindrical recess 32 of a depth Substantia-lly equal to three-eighths of the wavelength source I I, and the member 3 I, along a substantially one-eighth wavelength portion 33 thereof, is turned down to a reduced diameterwhich may be equal to the outer diameter of the axial extension 26 of member 24.

A further conductor such as a stii wire element 35 is seated in a suitable socket 36 in the member 24 and a suitable opening 31 in the mem- 3l', and serves as an electrical conductive connection between these points. Conductor `35 is connected to the inner conductor vI-of coaxial .line I4 at a junction 4I.

The transformer described above form's two coaxial transmission-line sections three-eighths wavelength long and two further coaxial transmission-line sections one-eighth wavelength long, each having an input end at the middle of transformer II and being short-circuited at its opposite end. lIhe rst three-eighths wavelength transmission-line section, designated I, comprises 'the outer cylindrical surface of' the axial-extension 2t of member 24 cooperating, as an inner conductor, with the portion of the tubular outer conductor I3 extending to the left of the coaxial line I4. The short-circuiting connection between inner conductor 25 and outer conductor I8 of this three-eighths wavelength transmission-line section is provided by the disc end 25 of the member 24. The second three-eighths wavelength transmission-line section, designated'II, comprises the portion of conductor 35 extending to the right of inner conductor I2, cooperating with the cylindrical bore 32 in the member 3I which forms the outer conductor therefor.

The third coaxial transmission-line section III, characterized by a length equal to one-eighth of the wavelength of source II, comprises the portion of the inner conductor 35extending to the left of the junction 4I, in cooperation with the cylindrical bore 2'! in the extension26 of the member 24. The fourth coaxial transmissionline sectionIV has as its inner conductor the reduced-diameter portion 33 of the member 3|, in cooperation with a coextensive `portion of the outer conductor I8.

It will be notedthat the'input end of coaxial transmission-line section III is connected Vbe- CTI .mission line I4 is connected to the junction be- 'tween the outer conductor of transmission-line section Iand the outer conductor of the oneeighth wavelength transmission-line section IV. Also, the inner conductor I2 of the coaxial line I4 is connected tothe junction 4I of the inner conductors of transmission-line section II and the .one-eighth' wavelength transmission-line section III. A rst output terminal of the transformer Il' is provided by the junction between the inner conductor of section I and the outer conductor of section III, this junction being the end of the axially extending portion 26 of member 24. AAs shown, one of the quarter-wavelength elements I5 of the dipole antenna I5, I6 lmay be connected, as by a soldered joint, to this annular junction between sections Iand III. A second output-terminal of transformer II is the similar annular junction between the outer conductor of transmission-line section II and the inner conductorci vtransmission-line -section IV, to which is connected the other-quarter-wavelength element I5 of antenna I5,'I.

The operation of the apparatus lshown in Figs. 1 and 2 is better understood by reierencef to the equivalent circuits in Figs. 3 and 4. In Fig.' 3 is shown a transformer -substantially .equivalent to vthat illustrated infthe sectional view of Fig. 1,'but

comprising three-eighths wavelength two-wire or parallel-conductor transmission-line sections Iv and II' and one-eighth-wavelength parallel-con- `fcluctor transmission-line sections III and IV.

The transmission-line sections I', II', III' and IV are'arranged with their input ends closely adjacent in a compact configuration, andare short-circuitedat their remote ends Vbyhconnec- The one-eighth wavelength transmission-line section-III is connected between a rstinput terminal of the transmission-line secytion I anda first input terminal of the-transpoints'i and 46 between transmission-linesections I and III and sections II and IVQ respectively, and a secondpairfof opposite junction points 4I and 48 between transmission-linesections I and IV--and sections II and III', respectively.

The unbalanced source Il', which mayhave one terminal49 connectedto ground, `is` connected to one pair of opposite junctions '4I `and l48 to deliver energy to the'transmission-line bridge transformer I'I',` while the other pair of opposite junctions 45 and 46 is connected to a balanced load `V5I, here represented as a resistor of which a centeretap or .midpoint "could be coupled through a leakage path to ground.

By comparison of Fig. ,i with Fig. 3, wherein corresponding transmission-line sections bear similar Roman numeral designations, except that those of Fig. 3 are given a prime mark, the circuit interconnection of the four transmissionline sections embodied within the transformer I1 is made readily apparent.

The operation of the transformers of Figs. 1 and 2 and Fig. 3 is best understood by reference to Fig. 4, wherein is shown a bridge circuit embodying capacitor I, inductor III, capacitor II" and inductor IV connected consecutively in clockwise order around a ring circuit and forming two pairs of opposite junctions 45', 4S and 41', 48 of a bridge.

An unbalanced or asymmetrical source II" is shown connected to one pair of opposite junctions 41 and 48', while a balanced load 5I is shown connected to opposite junctions 45' and 46. As is well known, a short-circuited transmission-line section one eighth wavelength long presents an inductive reactance equal in magnitude to the characteristic impedance of the transmission line. Similarly, a short-circuited transmission-line section three-eighths wavelength long presents an input impedance which is capacitive and which is equal in magnitude to the characteristic impedance of the transmission line. Thus, the equivalence of the bridge circuit arrangement shown in Fig. 4 with the transformers shown in Figs. 1 and 2 and Fig. 3 will be readily appreciated.

Preferably, the ratio of diameters of the inner and outer conductors of coaxial transmissionline sections I and IV in Fig. 1 is made equal to the diametral ratio of the inner and outer conductors of coaxial transmission-line sections II and III. The impedances presented at the input terminals of these short-circuited transmissionline sections are then all reactances of equal magnitude, those of sections I and II being capacitive, and thus of opposite sign to the inductive reactances of sections III and IV. Accordingly, adjoining transmission-line sections I and III form a rst series-resonant circuit connected across transmission line I4, and sections II and IV form a second series-resonant circuit similarly connected.

From a study of the equivalent diagram of Fig. 4, it is readily seen that if reactances I, II", III" and I are all of equal magnitude, then the currents flowing through all of the reactance elements will be of equal magnitude. The voltages e1, en, em and erv across the reactance elements are of equal magnitude also. As seen in Fig. 4A, the vector sum of voltages er" and ein" is equal to the vector voltage es applied by the source II". Similarly, the vector sum of voltages erv" and en" is equal to the source voltage es. 'Ihe voltage er. across load 5I' is given by the difference of the vector voltages across two reactance elements adjacent one of the source terminals, e. g., the difference of the voltages enrn and e1" across inductance IV and capacitance I". This is seen in Fig. 4A, where Thus, by the equivalence of the diagram in Fig. 4 to the transformers of Figs. 1, 2, and 2A and Fig. 3, it is seen that the transmission line transformers provide balanced output from an unbalanced source, or, more generally, serve to couple an unbalanced device to a balanced device.

It should be noted that while Figs. 1 and 2 illustrate the use of the transformer I1 as a coupling device between an unbalanced source and a balanced load, this coupling device is equally useful for coupling a balanced source to an unbalanced load. For example, if the dipole antenna I5, I6 is employed for receiving signals from a distant transmitting station, and the coaxial transmission line I4 is connected to the unbalanced input terminals of a receiver, the transformer I1 then functions for supplying energy from a balanced source (antenna I5, I6) to an unbalanced load. Similarly, the source I I in Fig. 3 and I I" in Fig. 4 may be replaced by a suitable energy utilization device, which may be grounded at one terminal thereof, and the balanced load 5I or 5I may be replaced by a balanced source, the transformer I1 or I1' remaining effective as a coupling device without tending to impair the balanced condition of the source.

In Fig. 5 is shown an arrangement generally similar to that shown in Figs. 1 and 2, with the exception that a two-wire shielded cable 5 I, having two inner conductors 62 and IES enclosed within an outer shielding tubular conductor S4, is shown joined to the transformer I1 and coupled thereby to the coaxial transmission line I4.

It will be noted that one conductor $2 of the two-wire transmission line 6I is connected to the junction between coaxial transmission-lino sections I and III, while the other conductor c3 is connected to the junction between coaxial trans- .mission-line sections II and IV. Thus, the conductors t2 and t3 are connected in place of quarter-wave elements I5 and I of the dipole antenna illustrated in Figs. 1 and 2.

The shielding conductor S4 may be joinedL to the tubular outer conductor I8 of transformer I1 at an opening opposite the junction of the conductor I3 with conductor I3. Either transmission eline ti or transmission line I4 may be coupled to an energy utilization device, the other transmission line being coupled to a suitable source of energy. Thus, the transformer I1 is suitable for coupling an unbalanced source connected to transmission line I4 to a balanced load connected to transmission line 5 I, or for coupling a balanced source connected to line 6I to an unbalanced or asymmetrical :load connected to transmission line I4.

As one illustration of the operation of the structure shown in Fig. 5, for example, a receiver 1I may be connected to the inner and outer conductors I2 and I3 of the coaxial transmission line I4, and a dipole receiving antenna 'I2 may be connected to the conductors 62 and S3 of the transmission line 6I.

It will be readily apparent that the transmission line sections I, II, III and IV could be made in other lengths than those illustrated without departure from the principles of the present invention. For example, sections I and II could be made i'lve-eighths wavelength long and sections III and IV could be made three-eighths wavelength long; or sections I and II could be made seven-eighths wavelength long and sections III and IV could be made iive-eighths wavelength long. Thus the lengths of the transmission-:line sections may be chosen from a wide variety which are known to provide reactive input impedance and accordingly to satisfy the requirements of an inductance-capacitance bridge as illustrated in Fig. 4.

As many changes could be made in the above construction and many apparently widely different embodiments of this invention could be cluding means therein providing a low-impedance current conduction path between said rst tubular conductor and said second tubular conductor, said first tubular conductor having an open end an appreciable distance from the said current conduction path means in said first and second transmission line sections, and means for supplying radio frequency potential between said inner conductor and said second tubular conductor at points thereon adjacent said open end of said rst tubular conductor, the length of the transmission path between said open end of said rst tubular conductor and one of said current conduction path means being shorter than an odd number of quarter wave-lengths by an amount appreoiabiy less than one-quarter wavelength, and the length of the transmission path between said open end of said first tubular conductor and the other of said current conduction path means being greater than an odd number of quarterwavelengths by an amount appreciably less than one-quarter Wavelength.

REFERENCE S CITED The following references are of record in the le of this patent:

. UNITED STATES PATENTS Number Name Date 2,107,025 Buschbeck, et al. Feb. 1, 1938 2,222,169 Buschbeck et a1. Nov. 19, 1940 2,286,179 Lindenblad June 9, 1942 2,318,237 Lindenblad May 4, 1943 2,327,418 Goldman Aug. 24, 1943 2,416,790 Barrow Mar. 4, 1947 2,416,981 Crosby Mar. 4, 1947 

